Unless complex scene illumination is used, video images frequently suffer from unrealistic reproduction of high-contrast motifs. Even the use of special reproduction characteristics (knee circuit) on the camera side corrects the described defect only to an insignificant extent. According to the invention, it is intended to solve the problem by electronic brightening of the shaded zones of the video image in the camera as early as during recording. At the same time, it is important to maintain the detail contrast even in the brightened regions.
A blurred diffuse image (mask) of the motif is produced on the image sensor in the camera by the objective and the electronically alterable diffusion element, and is stored in a frame store. This blurred image represents the rough distribution of brightness and darkness in the motif. The black and white contrast of the mask is determined in the evaluation circuit, and a control variable is generated pixel-by-pixel from this. The focused (color) video image which is subsequently produced on the image sensor when the diffusion element is switched to be crystal-clear is read and is amplified (or attenuated) pixel-by-pixel via the amplifier, which is controlled by the evaluation unit, such that the shadowed parts are brightened and/or the bright zones are attenuated.
The invention applies to the production of still video, movie video as well as in television cameras.
The invention relates to a method for electronic correction of the large-area contrast in video images as early as during recording.xe2x80x94As in the case of most picture reproduction methods, there is also a difficulty in screen reproduction of imaging objects with high-contrast light and shaded parts with a balanced brightness distribution, that is to say reproducing light areas and shadowed areas in a gradation as normally perceived. In addition, particularly with respect to the viewing of a screen, screens are normally used in reduced ambient lighting, which additionally makes it harder to reproduce shadowed parts such that they are transparent and have good detail. To avoid these weaknesses, it is necessary to aim for exactly matched scene illumination when making video recordings. This applies not only to studio work; even in the case of many professional recordings in the open air, it is necessary to work, when possible, with spotlights and fillers, which requires the use of a large number of personnel and is thus costly. However, on the other hand, artificial lighting is impracticable not only for many live reports, but also for amateur video recordings in the open air. Unnaturally high large-area contrasts are frequently created during video replay, as a consequence.
The aim of the invention proposed here is thus to describe a method using which the problems described above can be reduced. The physical principles of this method will be explained first of all.
Although efforts are being made to implement an image sensor characteristic which is curved in an S shape like a film (knee circuit) in a television camera, in order to make it possible to reproduce the most varied motif contrasts to a certain extent in a balanced manner with its aid, this method provides not only [sic] satisfactory results as well. The shadowed parts, which act in an unnaturally dark manner, and/or excessively bright light areas still occur in high-contrast motifs. The flat branches of the image sensor characteristics in the light and dark regions also lead to undifferentiated, low-contrast reproduction of details in these brightness regions.
To avoid this defect, it is proposed to introduce in video technology as well a method which is based on similar principles to those used in photography. There, when copying very high-contrast negatives, it is possible to brighten dark shadowed parts by means of the so-called dodging technique, without any loss in detail contrast occurring in so doing. This is done by the darkroom technician producing weaker exposure of the shadowed zones, by means of defined blurring, during the exposure of the photographic paper by moving his finger over the photographic paper in a local movement in the form of a fan, the shadowed zones in this way being brightened in the copy. This technique is, of course, dependent on correspondingly long exposure and large-format enlargement. It is obvious that considerable experience is required to carry out this technique, in order to avoid undesirable artifacts in so doing. The detail contrast (as well as the color contrast) are in this case maintained both within the shadowed zone to be brightened and in all the rest of the image, which guarantees the brilliant reproduction of all the image parts which is so important for an acceptable image impression. Photographically, this situation can be explained by the fact that the photographic paper is effectively given 2 different gradations as a result of the masking method: a steeper gradation (namely the original gradation) for reproduction of higher spatial frequencies (=detailed structures) and brilliant colors, and a flatter gradation for the lower spatial frequencies, that is to say the large-area structures, such as the lighted and shadowed regions for example. An even more general solution of this situation can now be achieved by producing in each case one individual blurred black-white mask for a plurality of spatial frequencies that are important to the motif, using computer means. In this way, it is possible to produce an effectively increasingly flatter transmission characteristic for increasingly coarser structures in the motif, although said transmission characteristic does not adversely affect the luminosity of the colors. This allows the natural charm of the pictures to be further improved. The xe2x80x9cmultilayer maskxe2x80x9d to be formed for this purpose is one of the subjects of the invention in the present patent application (see method B, page 10 in this context).
This dodging method must in general not be equated to the alternatively conceivable use of a photographic paper having a flatter gradation. The large-area contrast could admittedly likewise be reduced by using such a paper, but the brilliance of the picture in terms of color and contrast would, however, in this way be flattened, and the image quality would in consequence be made considerably poorer.
According to the invention, it is now intended to use the masking method outlined above to improve the video images, to be precise during recording in the video camera. The methods presented in the following text are all based on the fundamental principle that a blurred black and white image of the recorded motif (=original imprint) is produced, and correction values are derived from this xe2x80x9cmaskxe2x80x9d which can be used to correct the focused video image, in such a way that the large-area contrast of light and shadowed parts of the image is reduced. A major aim in this case is that the correction take place at the same time as the recording, and fully automatically.